> I'm attaching a draft patch in case anyone wants to provide some
> feedback. Thanks!

I'm now attaching a final version of my patch.

I've tested the following commands with it, using `read-from-minibuffer'
with a buffer-local HIST:

- previous-history-element (M-p)
- next-history-element (M-n)
- previous-matching-history-element (M-r)
- next-matching-history-element (M-s)
- isearch-forward (C-s)
- isearch-backward (C-r)

They've all worked correctly and used only the buffer's local history
value.

Thanks!